the interferential stimulation is for pain control, not loosening muscles. the pads get criss-crossed in order to cause a signal interference so there can be a large cloverleaf-like area of treatment.
also, a physical therapist can't "fix him once and for all". first, you don't know why he's going in the first place. second, he goes on and off, and it seems like chiropractic care is helping him out just fine. third, people mainly come back to the chiropractor's office because they're trying to keep their spine healthy and prolong early degenerative changes. it's not just about the pain symptoms, it's also about looking out for your future and making sure you don't end up the slow, elderly person whose hunched over you see shuffling along at the mall.
physical therapists are great at treating globally. they're garbage when it comes to treating segmentally.
if you're still coming into the chiropractic clinic without relief and the treatment plan isn't tapering off, then you're seeing the wrong chiropractor. also, if they aren't using therapy exercises to strengthen the weakened intrinsic muscles to problem areas to make them more stables so your adjustment would be more successful sooner, then you're probably not seeing the right guy.
edit: back to your original question, op. if you're going to get one, get a lsi system ii or chattanooga legend (xt or non-xt). i don't recommend it... i advise you to let someone who knows what their doing do it for you.